The present invention relates to air-conditioning apparatuses having a refrigerant circuit including a condenser, a pressure reducing device, an evaporator, and a variable displacement compressor, and, more particularly, to procedures for controlling the displacement of a variable displacement compressor installed in a vehicle air-conditioning apparatus.
A typical air-conditioning apparatus for a vehicle has a refrigerant circuit including a condenser, an expansion valve (pressure reducing device), an evaporator, and a compressor. The evaporator sends refrigerant gas to the compressor by which the gas is compressed and sent to the condenser. The evaporator exchanges heat between the refrigerant flowing in the refrigerant circuit and the air drawn in a passenger compartment. That is, heat is transferred from the air passing through the evaporator to the refrigerant flowing through the evaporator. The amount of the transferred heat varies in accordance with the heat load, or the cooling load. The pressure of the refrigerant gas at the outlet of the evaporator or downstream from the evaporator thus reflects the amount of the cooling load. A variable displacement compressor, which is often used in vehicles, has a displacement control mechanism. The control mechanism maintains the pressure at the outlet of the evaporator (suction pressure Ps) at a predetermined target value (target pressure Pset). Specifically, the displacement control mechanism executes a feedback control program for adjusting the displacement of the compressor to alter the inclination angle of a swash plate, in accordance with the suction pressure Ps, so that the displacement corresponds to the cooling load. The displacement control mechanism typically includes a displacement control valve, or an internal control valve. The internal control valve has a pressure sensitive member such as a bellows or diaphragm for sensing the suction pressure Ps. The valve further includes a valve body that is positioned in accordance with movement of the pressure sensitive member. The valve body varies the opening size of the internal control valve to adjust the pressure in a chamber accommodating the swash plate (crank chamber). The inclination angle of the swash plate is thus determined.
However, if the target pressure Pset is fixed, or if the internal control valve is not capable of varying the target pressure Pset, the desired cooling cannot be performed accurately. Thus, the internal control valve is configured to vary the target pressure Pset through an external electric control procedure. Specifically, the internal control valve includes an actuator such as a solenoid. The solenoid applies force to the pressure sensitive member, which determines the target pressure Pset. The target pressure Pset is altered by varying the force applied the solenoid through the external control procedure.
A compressor installed in a vehicle is normally actuated by power transmitted from the vehicle""s engine. The compressor consumes a considerable amount of the engine power and increase the load on the engine. Accordingly, when the vehicle needs additional engine power, such as when the vehicle is being accelerated or ascending a sloped surface, the compressor displacement is minimized to reduce the load acting on the engine due to operation of the compressor. For example, the aforementioned variable displacement compressor adjusts the target pressure Pset to a value higher than the current target pressure Pset by means of the internal control valve having the solenoid. The current suction pressure Ps becomes thus relatively low as compared to the adjusted target pressure Pset. This minimizes the compressor displacement to minimize the load acting on the engine due to the operation of the compressor.
However, an analysis regarding the operation of the variable displacement compressor indicates that the above feedback control procedure does not always reduce the load acting on the engine sufficiently, if the target pressure Pset is adjusted in accordance with the suction pressure Ps.
FIG. 11 is a graph showing the relationship between the suction pressure Ps and the compressor displacement Vc. In the graph, the relationship is represented by curves. That is, the relationship between the suction pressure Ps and the compressor displacement Vc varies in accordance with the heat load acting on the evaporator. Thus, when the target pressure Pset is set to a pressure Ps1, the displacement achieved automatically by the internal control valve is varied within a certain range (for example, xcex94vc) in relation to the amount of the heat load acting on the evaporator.
For example, even though the target pressure Pset is raised in accordance with the current suction pressure PS, the displacement Vc is not lowered enough to reduce the load acting on the engine if the heat load on the evaporator is very high. In other words, as long as the target pressure Pset is adjusted in accordance with the current suction pressure Ps, the compressor displacement is not adjusted quickly to the desired value when the heat load acting on the evaporator is great.
As described above, as long as there is no need for reducing the load acting on the engine, controlling of the compressor displacement in accordance with the suction pressure Ps, which reflects the heat load on the evaporator, achieves the objective of the air-conditioning apparatus. That is, the air-conditioning apparatus maintains the temperature in the passenger compartment at a comfortable level regardless of the temperature of the ambient air. However, when the load acting on the engine must be reduced, the compressor displacement cannot be reduced quickly if it is controlled in accordance with the suction pressure Ps.
Accordingly, it is an objective of the present invention to provide an air-conditioning apparatus that quickly varies displacement of a compressor through an external control procedure when necessary, regardless of the heat load acting on an evaporator. It is also an objective of the present invention to provide a procedure for controlling a variable displacement compressor to adjust its displacement for maintaining the temperature in a passenger compartment at a comfortable level when there is no need to reduce the load acting on the engine and to quickly lower the displacement when the load acting on the engine must be reduced.
To achieve the above objective, the present invention is an air-conditioning apparatus having a refrigerant circuit including a condenser, a pressure reducing device, an evaporator, and a variable displacement compressor. The air-conditioning apparatus has a torque detecting device, an external information detecting device, and a control device. The torque detecting device directly or indirectly detects torque acting on the compressor during operation of the compressor. The external information detecting device detects various external information other than the torque. The control device determines a target torque in accordance with the external information provided by the external information detecting device. The control device executes a feedback control program for displacement of the compressor such that the torque detected by the torque detecting device approaches the target torque.
As described above, the air-conditioning apparatus of the present invention controls the compressor displacement in accordance with the torque acting on the compressor. Specifically, the feedback control program is performed for the compressor displacement such that the torque detected by the torque detecting device approaches the target torque, which is determined by the control device in accordance with the external information. In other words, the compressor displacement is controlled regardless of a physical magnitude, such as suction pressure, reflecting the heat load acting on the evaporator. The control of the compressor displacement is thus performed only in accordance with the torque acting on the compressor. Accordingly, the compressor displacement is varied quickly to reduce the torque acting on the compressor when necessary. As a result, although the feedback control procedure of the present invention maintains the passenger compartment temperature at a comfortable level when the vehicle is operated in a normal state, the feedback control procedure quickly varies the compressor displacement when the load acting on the engine must be reduced.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.